<refentry xmlns="http://docbook.org/ns/docbook" version="5.0"
xml:id="man.tracepath">

  <refentryinfo>
    <title>tracepath</title>
    <productname>iputils</productname>
  </refentryinfo>

  <refmeta>
    <refentrytitle><application>tracepath</application></refentrytitle>
    <manvolnum>8</manvolnum>
    <refmiscinfo class='manual'>iputils</refmiscinfo>
  </refmeta>

  <refnamediv>
    <refname><application>tracepath</application></refname>
    <refpurpose>traces path to a network host discovering MTU along
    this path</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <cmdsynopsis sepchar=" ">
      <command>tracepath</command>
      <arg choice="opt" rep="norepeat">
        <option>-4</option>
      </arg>
      <arg choice="opt" rep="norepeat">
        <option>-6</option>
      </arg>
      <arg choice="opt" rep="norepeat">
        <option>-n</option>
      </arg>
      <arg choice="opt" rep="norepeat">
        <option>-b</option>
      </arg>
      <arg choice="opt" rep="norepeat">
        <option>-l
        <replaceable>pktlen</replaceable></option>
      </arg>
      <arg choice="opt" rep="norepeat">
        <option>-m
        <replaceable>max_hops</replaceable></option>
      </arg>
      <arg choice="opt" rep="norepeat">
        <option>-p
        <replaceable>port</replaceable></option>
      </arg>
      <arg choice="opt" rep="norepeat">
        <option>-V</option>
      </arg>
      <arg choice="req" rep="norepeat">destination</arg>
    </cmdsynopsis>
  </refsynopsisdiv>

  <refsection>
    <info>
      <title>DESCRIPTION</title>
    </info>
    <para>It traces the network path to
    <emphasis remap='I'>destination</emphasis> discovering MTU along
    this path. It uses UDP port
    <emphasis remap='I'>port</emphasis> or some random port. It is
    similar to
    <emphasis remap='B'>traceroute</emphasis>. However, it does not
    require superuser privileges and has no fancy options.</para>
    <para>
    <emphasis remap='B'>tracepath -6</emphasis> is a good replacement
    for
    <emphasis remap='B'>traceroute6</emphasis> and classic example
    of application of Linux error queues. The situation with IPv4
    is worse, because commercial IP routers do not return enough
    information in ICMP error messages. Probably, it will change,
    when they are updated. For now it uses Van Jacobson's
    trick, sweeping a range of UDP ports to maintain trace
    history.</para>
  </refsection>

  <refsection>
    <info>
      <title>OPTIONS</title>
    </info>
    <variablelist remap='TP'>
      <varlistentry>
        <term>
          <option>-4</option>
        </term>
        <listitem>
          <para>Use IPv4 only.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>
          <option>-6</option>
        </term>
        <listitem>
          <para>Use IPv6 only.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>
          <option>-n</option>
        </term>
        <listitem>
          <para>Print primarily IP addresses numerically.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>
          <option>-b</option>
        </term>
        <listitem>
          <para>Print both: Host names and IP addresses.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>
          <option>-l</option>
        </term>
        <listitem>
          <para>Sets the initial packet length to
          <emphasis remap='I'>pktlen</emphasis> instead of 65535 for
          <emphasis remap='B'>IPv4</emphasis> or 128000 for
          <emphasis remap='B'>IPv6</emphasis>.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>
          <option>-m</option>
        </term>
        <listitem>
          <para>Set maximum hops (or maximum TTLs) to
          <emphasis remap='I'>max_hops</emphasis> instead of
          30.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>
          <option>-p</option>
        </term>
        <listitem>
          <para>Sets the initial destination port to use.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>
          <option>-V</option>
        </term>
        <listitem>
          <para>Print version and exit.</para>
        </listitem>
      </varlistentry>
    </variablelist>
  </refsection>

  <refsect1 id='output'>
    <title>OUTPUT</title>
    <literallayout remap='.nf'>
root@mops:~ # tracepath -6 3ffe:2400:0:109::2
 1?: [LOCALHOST]                              pmtu 1500
 1:  dust.inr.ac.ru                   0.411ms
 2:  dust.inr.ac.ru        asymm  1   0.390ms pmtu 1480
 2:  3ffe:2400:0:109::2               463.514ms reached
     Resume: pmtu 1480 hops 2 back 2
    </literallayout>
    <para>The first column shows the TTL of the probe, followed by
    colon. Usually the value of TTL is obtained from the reply from
    the network, but sometimes it does not contain the necessary
    information and we have to guess it. In this case the number is
    followed by ?.</para>
    <para>The second column shows the network hop which replied to
    the probe. It is either the address of the router or the word
    [LOCALHOST], if the probe was not sent to the network.</para>
    <para>The rest of the line shows miscellaneous information about
    the path to the corresponding network hop. It contains
    the value of RTT, and additionally it can show Path MTU when it
    changes. If the path is asymmetric or the probe finishes before
    it reaches the prescribed hop, the difference between number of
    hops in forward and return direction is shown next to the
    keyword "async". This information is not reliable, e.g. the
    third line shows asymmetry of 1. This is because the first probe
    with TTL of 2 was rejected at the first hop due to Path MTU
    Discovery.</para>
    <para>The last line summarizes information about all the paths
    to the destination. It shows detected Path MTU, amount of hops
    to the destination and our guess about the number of hops from
    the destination to us, which can be different when the path is
    asymmetric.</para>
  </refsect1>

  <refsect1 id='see_also'>
    <title>SEE ALSO</title>
    <para>
    <citerefentry>
      <refentrytitle>traceroute</refentrytitle>
      <manvolnum>8</manvolnum>
    </citerefentry>,
    <citerefentry>
      <refentrytitle>traceroute6</refentrytitle>
      <manvolnum>8</manvolnum>
    </citerefentry>,
    <citerefentry>
      <refentrytitle>ping</refentrytitle>
      <manvolnum>8</manvolnum>
    </citerefentry>.</para>
  </refsect1>

  <refsect1 id='author'>
    <title>AUTHOR</title>
    <para>
    <command>tracepath</command> was written by Alexey Kuznetsov
    &lt;kuznet@ms2.inr.ac.ru&gt;.</para>
  </refsect1>

  <refsect1 id='security'>
    <title>SECURITY</title>
    <para>No security issues.</para>
    <para>This lapidary deserves to be elaborated.
    <command>tracepath</command> is not a privileged program, unlike
    <command>traceroute</command>,
    <command>ping</command> and other beasts of their kind.
    <command>tracepath</command> may be executed by everyone who has
    enough access to the network to send UDP datagrams to the
    desired destination using the given port.</para>
  </refsect1>

  <refsect1 id='availability'>
    <title>AVAILABILITY</title>
    <para>
    <command>tracepath</command> is part of
    <emphasis remap='I'>iputils</emphasis> package.</para>
  </refsect1>
</refentry>
